It's me again. Two years later on, I approached the issue again. I tried everything I tried before as well as some new ideas. Nada. Nyet. Nothing.
Finally I checked out Libre Office Impress out of desperation. And it worked. So, I will complete the lesson with Impress, although it's a struggle. I've always found Libre Office slow and clunky compared to Softmaker's office apps. It's going to take me twice as long to prepare the lesson, moaning and complaining every inch of the way. And I will gratefully return to Presentations when this task is finished.
This is not a complaint aimed at Softmaker, by the way. It seems there is little interest in adding this feature, and to be honest I don't expect to ever use it again. I wrote this post just in case somebody else has a similar problem.